Opiate receptors solubilized with the detergent, CHAPS, are macromolecular complexes carrying at least two activities: reversible binding of opiate ligands, and a guanine nucleotide (and Na+) sensitive regulatory activity. In membranes prepared from neuroblastoma x glioma hybrid cell NG108-15 or rat striatum, opiate receptors inhibit adenylate cyclase activity by stimulating GTP hydrolysis catalyzed by a specific, low Km, GTPase. These observations provide a general mechanism for the action of opiate receptors and for other inhibitory receptors coupled to adenylate cyclase. Opiate receptors have been transferred from one membrane to another by fusion techniques and are specifically labelled by the appropriate affinity ligands.